Transition metal complexes trifluoroacetates

Figure 12. Supramolecular porphyrins obtained by coordination of transition metal complexes to the periphery of the macrocyclic ring (a similar scheme can be extended to the 3-TPyP and analogous porphyrazine species), [dmso = dimethyl sulfoxide (ligand), form = A,A-9-di- -tolyformamidinate, and TFA = trifluoroacetate.l...

Stereoselective oxycarborative addition is also achieved in cycloaddition and cyclooligomeriza-tion reactions. Thus, hetero-Diels-Alder reactions of dienes and aldehydes are not only catalyzed by main group Lewis acids, but also by transition metal complexes 10°. Tris[3-(heptafluoropropyl-hydroxymethylene)-( + )-camphorato]europium [( + )-Eu(hfc)3] and similar vanadium complexes have been used as the chiral catalyst in [4 + 2] cycloadditions of various achiral and chiral dienes to aldehydes63 67-101. With achiral silyloxydienes only moderate asymmetric inductions are observed, however, with chirally modified dienes, high double diastereoselectivities are achieved. Thus, the reaction of benzaldehyde with 3-terf-butyldimethylsilyloxy-l-(/-8-phenvl-menthoxy)-l.3-butadiene (1) gives (2/ .6/ )-4-wf-bntyldimethylsilyloxy-5,6-dihydro-6-phenyl-2-[(17 ,3/ ,45 )-8-phenylmenthoxy]-2f/-pyran (2) in 95% yield with a diasteieoselectivity of 25 1 ss. After crystallization and hydrolysis with trifluoroacetic acid, optically pure (2/ )-2,3-di-hydro-2-phenyl-4-(4//)-pyranone (3) is obtained in 87% yield. 

Shilov reported some of the earliest evidence that transition metal complexes could selectively cleave the C-H bonds of alkanes in a catalytic fashion. Shilov showed that H/D exchange would occur between alkanes and deuterated acid in the presence of platinum complexes (Equation 18.5 and Table 18.1). In addition, Shilov showed that the oxidation of alkanes occurred in the presence of a platinum(II) catalyst, although a platinum(IV) complex was needed as the oxidant. These reactions led to a mixture of alkyl halides formed from the halide of the Pt(IV) oxidant (Equation 18.6) and trifluoroacetate from the trifluoroacetic acid solvent. The cost of platinum(IV) as an oxidant makes this reaction impractical. However, these results provided hope that selective alkane functionalization could be developed because H/D exchange occurred faster at primary C-H bonds than at secondary C-H bonds (Table 18.1), and some selectivity for oxidations of primary C-H bonds over secondary C-H bonds was observed. As noted in Chapter 6, these results motivated a large number of groups to seek transition metal complexes that would insert into, or by other means selectively cleave, the C-H bond of alkanes and create products from this bond cleavage that could be observed directly. 

Compound [Cr(C0)g(02CCF3)] may be prepared by oxidative substitution of Ag(02CCF3) or Hg(02CCF3)2 on [Cr2(CO)io] (218). Transition metal hydrido complexes have been obtained by oxidative addition reactions of trifluoroacetic acid which often lead to the formation of trifluoroacetato complexes. Equation (3) is an example of such a... 

Trifluoroacetato complexes of transition metals have also been obtained from oxidative elimination reactions in which neutral tt-acceptor ligands have been displaced in reactions with trifluoroacetic acid. These reactions may lead to hydrido complexes [Eqs. (6) (253) and (7) (26)] or they may not [Eqs. (8) (39) and (9) (19)]. 

Direct C-H activation of hydrocarbon by means of transition metals has also been explored. Cyclohexane reacted with Pd(OAc)2 in the presence of potassium persulfate-trifluoroacetic acid under CO pressure and produced the desired cyclo-hexanecarboxylic acid in low yields and TON (eq. (13)). The electrophilic carbox-ylation is explained by the change of Pd(OAc)2 to Pd(OCOCp3)2 in trifluoroacetic acid as solvent. Electrophilic attack on a C-H bond should give an alkyl Pd complex. CO insertion followed by reductive elimination affords a reactive mixed anhydride which was detected before workup. 

Transition metal catalysed prenylation. There is a new one-step N-tert-prenylation of indole developed by Baran and co-workers [42] which still outcom-petes the chemoenzymatic approach (Scheme 5). Isobutene (21) as prenyl source is reacted with side-chain Fmoc-protected tryptophan methyl ester 20 in the presence of catalytic amounts of Pd(OAc)2 and superstoichiometric amounts of Ag(I) trifluoroacetate and Cu(II) acetate. The protocol also requires the presence of oxygen. After about 1 day at 35°C, the N-tert-prenylated indole is obtained in a yield of about 60%. The mechanism has not been elucidated, but may involve a 7i-allyl-Pd(II) complex which is coordinated by the indole nitrogen or by C3. In the latter case, a Pd-Claisen rearrangement of a 3-palladated indole would follow. Ag (I) functions as reoxidant of Pd(0). 

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